Transmitter with Improved Sensitivity and Shielding

ABSTRACT

A magnetic field transmitter, especially a transmitter used in conjunction with wireless communications earplugs. In one embodiment, a plate of magnetic material is used behind a coil of electrical conductor to improve the efficiency of the transmitter and to provide electrical and magnetic shielding. The specific dimensions and characteristics of the preferred embodiment of the transmitter described herein provide for efficient wireless communications.

REFERENCE TO RELATED APPLICATIONS

This is a continuation patent application of copending application Ser.No. 13/796,480, filed Mar. 12, 2013, entitled “Transmitter with ImprovedSensitivity and Shielding”, which claimed benefit under 35 USC §119(e)of provisional application No. 61/694,481, filed Aug. 29, 2012, entitled“Transmitter with Improved Sensitivity and Shielding”. Theaforementioned applications are hereby incorporated herein by reference.

ACKNOWLEDGMENT OF GOVERNMENT SUPPORT

This invention was conceived during a Navy Phase 2.5 SBIR contractN68335-09-C-0003. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention concerns magnetic field transmitters, especiallytransmitters that are used in conjunction with wireless communicationsearplugs.

2. Description of Related Art

The inventor's co-pending U.S. patent application Ser. No. 11/837,129,entitled “Wireless Communications Headset System Employing a LoopTransmitter that Fits Around the Pinna” describes a loop transmittercomprising a coil of wire having an open center sized to fit around apinna of a user's ear for use with communications earplugs. Thisinvention is very effective in situations where there is limited volumeavailable behind the human pinna such as when the transmitter is locatedin a shallow headset earcup or helmet, or other such applications, andthe transmitter would otherwise be in contact with the pinna due to lackof space. However, larger transmitter coils are less efficient comparedto smaller transmitters because they generate more unused magnetic flux.High efficiency transmitters are desirable in battery-operated devicesbecause these devices would run longer on a single charge. Therefore,when space is available for a particular loop transmitter, it isdesirable in many applications to employ a higher efficiency looptransmitter.

SUMMARY OF THE INVENTION

This invention concerns magnetic field transmitters, especiallytransmitters that are used in conjunction with wireless communicationsearplugs. In one embodiment, a plate of magnetic material is used behinda coil of electrical conductor to improve the efficiency of thetransmitter and to provide electrical and magnetic shielding. Thespecific dimensions and characteristics of the preferred embodiment ofthe transmitter described herein provide for efficient wirelesscommunications.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1A shows a round coil magnetic field transmitter.

FIG. 1B shows a cross-sectional view of a portion of the embodimentshown in FIG. 1A.

FIG. 1C shows another cross-section view of a portion of the embodimentshown in FIG. 1A and indicates magnetic field lines.

FIG. 2A shows an embodiment of the invention incorporating a magneticbacking plate.

FIG. 2B shows a cross-sectional view of the embodiment shown in FIG. 2Aindicating magnetic field lines.

FIG. 3A shows an embodiment of the invention incorporating a cup-shapedmagnetic backing plate.

FIG. 3B shows a cross-sectional view of the embodiment shown in FIG. 3Aindicating magnetic field lines.

FIG. 4A shows an embodiment of the invention incorporating a cup-shapedmagnetic backing plate with magnetic core.

FIG. 4B shows a cross-sectional view of the embodiment shown in FIG. 4Aindicating magnetic field lines.

FIG. 5 shows an embodiment of the invention incorporating a perforatedcup-shaped magnetic backing plate.

FIG. 6A shows an embodiment of the invention incorporating a perforatedcup-shaped magnetic backing plate with speaker.

FIG. 6B shows a cross-sectional view of the embodiment shown in FIG. 6A.

FIG. 7 shows an embodiment of the invention incorporating a perforatedcup-shaped magnetic backing plate with speaker mounted to the back ofthe backing plate.

FIG. 8A shows an embodiment of the invention incorporating a wirelesscommunications earplug.

FIG. 8B shows a cross-sectional view of the embodiment shown in FIG. 8A.

FIG. 9 shows an embodiment of the invention incorporating a wirelesscommunications earplug, speaker and core material and indicates magneticfield lines.

DETAILED DESCRIPTION OF THE INVENTION

This invention concerns magnetic field transmitters, especiallytransmitters that are used in conjunction with wireless communicationsearplugs. In one embodiment, a plate of magnetic material is used behinda coil of electrical conductor to improve the efficiency of thetransmitter and to provide magnetic and electrical shielding. The coilgeometry of the invention improves efficiency for wirelesscommunications with a wireless earplug.

A transmitter coil 2 is shown in FIGS. 1A through 1C. The coil 2 isshown in FIG. 1A with inner radius R1 and outer radius R2 and isconstructed using a continuous conductor 4 that creates loops of thecoil 2. By Ohm's Law, when a voltage v is applied to the ends of theconductor 4, a current i results, and, inversely, when a current i isapplied to the ends of the conductor 4, a voltage v results. The currenti flows into a first loop 9, followed by a second loop 11, followed byan inner front loop 7 and continues through all loops exiting after thelast loop 13 seen in FIG. 1B.

FIG. 1B is a cross section along the lines 1B-1B in FIG. 1A. The woundconductor 4 creates a conductor bundle 6 with annulus width L2 andannulus depth L1, where L2=R2-R1. In this example, the conductor bundlecross section indicates a coil 2 of three rows and four columnsindicating twelve loops. The conductor 4 has diameter D1.

FIG. 1C shows the cross section along the lines 1C-1C in FIG. 1A. Twocross sections of the conductor bundle 6 can be seen. Current i flowinginto conductor 4 in FIG. 1A results in magnetic field B and circulatingmagnetic field lines 5 (shown as dashed lines).

Magnetic field lines 5 are shown here as circular lines for illustrativepurposes; however, in reality they would have a more complicatedrectangular shape. Moreover, only six magnetic field lines 5 are shownwhereas there are infinitely many. The magnetic field lines 5 aresymmetric front to back. If the current i corresponds to acommunications signal, the magnetic field can be sensed by a wirelessreceiver and used for wireless communications.

Between the conductor bundle 6 cross sections, the magnetic field B isgenerally uniform and pointing in the same direction to the front. Themagnetic flux density generated by the inner front loop 7 of the coil 2at inner front loop center point c when placed in a vacuum is given byB=u_(o)i/(2 R1), where B is the magnetic flux density (in Teslas) andthe constant u_(o) is the permeability of free space (4π×10⁻⁷ N/A²).

The total magnetic flux density generated by all the loops of the coil 2at the center point c when placed in a vacuum is given by B=Nu_(o)i/(2Re) where N is the number of loops in the coil, and Re is the effectiveradius of the coils taken as a whole. Re can be approximated by addingall loop radii and dividing by the number of loops when the coildimensions L1 and L2 are small compared to R1. In this exampleembodiment there are twelve loops in the coil, so N=12.

The electrical impedance Z seen by a voltage source into the ends ofconductor 4 is Z=R+jwL, where R is the electrical resistance,j=sqrt(−1), w is the radian frequency, and L is the electricalinductance. The break frequency of the coil is defined as f_(b)=R/(2πL)in Hz. At frequencies below the break frequency resistive losses(manifested as heat) become increasingly higher and the coil becomesinefficient at generating magnetic fields.

The resistance R can be calculated using the formula R=ρ1/A, where ρ isthe resistivity of the conductor, 1 is the total length of the conductor4, A is the cross sectional area of the conductor 4 and A=π(D/2)².

The inductance of a single coil in a vacuum with radius R1 is given byL=(u_(o) πR1)/2 in Henries (H). For a coil of N loops and effectiveradius Re, the inductance is given by L=N² (u_(o) πRe)/2.

The imaginary power into a coil at frequencies substantially above thebreak frequency f_(b) can be approximated as P=i²wL. Hence, P=i²w N²(u_(o) πRe)/2. If the effective coil radius Re is doubled, the powerinto the coil doubles, for a given current i. However, the flux densityB is reduced by a factor of ½.

Higher imaginary power P requires greater actual power from batterysources and lower battery duration. Hence, all other parameters beingequal, it is desirable to minimize the size of a coil 2 when the goal isto minimize the imaginary power needed to generate a flux density withina coil 2. Break frequencies between 20 Hz and 2.0 kHz are appropriatedepending on the product type and design goals. Low break frequenciescan be used where weight is not a concern, whereas higher breakfrequencies may be appropriate for products such as headsets whereweight is critical.

FIG. 2A shows the coil of FIG. 1A with the addition of a magneticbacking plate 10, while FIG. 2B shows a cross section of FIG. 2A alongthe line 2B-2B. The magnetic backing plate 10 in FIG. 1A is attached inclose proximity to the back side of the coil 2.

FIG. 2B depicts the general effect of mounting the magnetic backingplate 10. The magnetic backing plate 10 tends to draw the magnetic fieldlines 5 by creating a low reluctance path. Note that the magnetic fieldline 5 geometries shown here are approximate and are for illustrativepurposes. In reality, there are an infinite number of field linesincluding lines that are more circular in geometry as the distance fromthe magnetic backing plate increases.

The effect of the magnetic backing plate 10 is to tend to project themagnetic field lines toward the front direction rather than the backdirection which is desirable in circumstances such as when a wirelesscommunications earplug is employed.

The result of adding the magnetic backing plate is to increase theinductance L of the coil, while the resistance R is unchanged. Thisdecreases the coil 2 break frequency f_(b) and improves the coil 2efficiency at low frequencies.

The magnetic backing plate 10 is constructed of a material exhibitinglow reluctance such as mu-metal or other low-reluctance magneticmaterials. The magnetic backing plate 10 must have sufficient thicknessT1 to prevent magnetic field saturation which would cause signaldistortion at high field strengths. However, if the magnetic backingplate 10 is made of a conductive material, excess thickness T1 can leadto eddy current losses. This can be prevented by using a generallynon-conducting material, such as magnetic ceramics, or by usinglaminated metal sheets with insulating material separating the layers.

In a test of the transmitter, a coil was constructed in the laboratoryusing 200 loops of 33 AWG conductor having a wire diameter of 0.00795 in(0.18 mm) The coil had an inner radius R1=1.40 in (35.6 mm), an outerradius R2=1.75 in (44.5 mm) and a thickness L1=0.075 in (1.9 mm) Themeasured inductance was 2.8 mH, while the resistance was 17 Ohms.

When a circular flat magnetic backing plate of radius 1.75 in (44.5 mm)and thickness 0.006 in (0.15 mm) and co-netic AA material was placedtightly behind the coil, the inductance increased to 3.9 mH, which is a39% increase over the coil without the backing plate. The magneticbacking plate weighed 0.075 oz (2.13 g) while the coil weighed 0.210 oz(5.95 g). The weight of the magnetic backing plate 10 can be reduced byas much as 50%, to 0.0375 oz (1.06 g) by making perforations through theplate, without appreciably affecting the inductance, as long as theperforations are made within radius R1. This can be done because themagnetic backing plate 10 has its strongest influence in the regionclosest to the coil 2.

To increase the inductance by 39% by adding loops to the coil, one wouldhave to add 18% more loops which would increase the weight by 18%. Thesame inductance increase using only a perforated magnetic backing platewould increase the weight by 18%.

Even though there is no weight advantage, in this example, whenincorporating the magnetic backing plate 10, the magnetic backing plate10 provides the benefit of electromagnetic shielding to protect the coilfrom electromagnetic radiation and to reduce radiated electromagneticradiation.

Preferably, the magnetic backing plate 10 is electrically connected to acircuit ground point to provide improved electrical shielding. Inparticular, if an electronic circuit board is employed in back of themagnetic backing plate 10, the magnetic backing plate 10 shunts themagnetic field. Otherwise, the magnetic field can interfere with theelectronic circuit board and eddy currents in the circuit board canpartially cancel the magnetic field reduction efficiency.

Further improvements in the transmitter efficiency can be provided byemploying a cup-shaped magnetic backing plate 16 as seen in FIGS. 3A and3B. A cup shape can be achieved by using means such as stamping ormolding. If the material is stamped, it may need to be heat treated torestore optimal magnetic properties.

An additional 16% increase in inductance can be achieved in this waycompared to the disc magnetic backing plate 10 seen in FIGS. 2A and 2B.Hence, in this example, the number of loops N of the coil 2 can bereduced to achieve the same inductance with a net reduction in weight.

The inductance of the embodiment shown in FIGS. 3A and 3B increasesbecause the cup-shaped magnetic backing plate 16 provides an improvedlow reluctance path for the magnetic field lines 5 around the coil 2.The magnetic field lines 5 tend to be drawn into the low reluctancecup-shaped magnetic backing plate 16. Note that the magnetic field line5 geometries shown here are approximate and are for illustrativepurposes. In reality, there are an infinite number of field linesincluding lines that are more circular in geometry as the distance fromthe cup-shaped magnetic backing plate 16 increases.

The cup-shaped magnetic backing plate 16 does not wrap completely aroundthe coil 2 because this would shunt the magnetic field lines 5completely around the coil 2 instead of toward the desired frontdirection.

FIGS. 4A and 4B show the embodiment of FIGS. 3A and 3B with the additionof magnetic core material 24. The core material 24 tends to shunt themagnetic field lines 5 in front of and away from the coil 2, which isdesirable. However, employing core material 24 doesn't provide as muchbenefit because the magnetic field lines 5 are already pointed in theforward direction. The core material is preferably made ofnon-conductive material, such as magnetic ceramic, because there is thepotential of significant eddy current losses if conductive metal wasused (such eddy currents could also be reduced by forming the core withradially oriented slots).

FIG. 5 shows an embodiment of the invention very similar to FIGS. 3A and3B; however, this embodiment employs multiple perforations 24 to createa cup-shaped magnetic perforated backing plate 22. The perforations 24help prevent sound from reflecting off the transmitter toward an ear orcausing a reflecting surface that can yield resonances if thetransmitter is used in a headset. The perforations reduce weight withoutreducing the inductance significantly. A single perforation, creating anannulus shape, can be used as well. However the magnetic and electricalshielding provided is diminished when using an annulus geometry.

The perforated backing plate 22 from FIG. 5 is employed in theembodiment shown in FIGS. 6A and 6B. The embodiment shown in FIG. 6A and6B is similar to the embodiment shown in FIG. 5 except that theembodiment shown in FIGS. 6A and 6B employs a speaker 37.

The speaker 37 could be a dynamic speaker, as shown in this embodiment,or could employ other types of construction such as piezoelectric,balanced armature or other type that produces sound. For claritypurposes, the electrical terminals of the speaker are not shown. Aninput electrical source is coupled to the electrical terminals ofspeakers to excite the speaker and create sound.

The speaker in this embodiment employs a housing 29 and a diaphragm 30that moves in response to current flowing through a voice coil 27 causedby an input electrical source. The static magnetic field generated by aspeaker magnet 34 and shunted by a speaker backing plate 32 creates aforce on the voice coil 27 due to current flow. To reduce back pressuredue to the diaphragm 30 movement, the speaker 37 employs vents 28. Fourvents 28 are indicated in FIG. 6A of this embodiment.

The perforations 24 of the perforated backing plate 22 allow backpressure generated by the speaker 37 to vent. This venting provides theadvantage of reducing restraining forces on the speaker diaphragm 30resulting in improved speaker efficiency compared to incorporating abacking plate that is not perforated.

The embodiment shown in FIG. 7 is similar to the embodiment shown inFIGS. 6A and 6B except that a speaker 40 is employed in back of thebacking plate 22 and a speaker 40 housing 42 is employed that provides aseparation space for the speaker diaphragm 30 to vibrate withoutcontacting the perforated backing plate 22. In this embodiment, theperforations 24 allow sound generated by the speaker 40 to pass throughthe perforated backing plate 22 to provide sound in the front direction.

FIGS. 8A and 8B are similar to the embodiment shown in FIG. 1A through1C except that a wireless communications earplug 50 is employed. Thewireless communications earplug 50 comprises a receiver coil 60 to sensethe magnetic field B generated by the transmitter coil 2. The receivercoil 60 is electrically coupled to an electronics circuit 62 forprocessing the sensed magnetic field to provide an appropriate signalfor coupling to an earplug speaker 64. The receiver coil 60, electroniccircuit 62 and earplug speaker 64 are contained in an earplug housing52. The speaker 64 is acoustically coupled to a sound delivery tube 58for coupling to a human ear canal (not shown). An ear tip 54 made ofmaterial such as foam, rubber, plastic or other materials is used tocreate an acoustic seal with an ear canal. The acoustic seal preventsacoustic noise from entering the human ear canal and provides a higheracoustic impedance for the earplug speaker 64 to improve the earplugspeaker 64 sensitivity.

FIG. 9 shows an embodiment that incorporates the wireless earplug 50shown in FIGS. 8A and 8B with the embodiment shown in FIG. 7 and aperforated core 70. The core is perforated in this embodiment to allowsound transmission toward the front direction.

In this embodiment, a user can choose between receiving communicationsfrom the transmitter speaker 40 without using a wireless communicationsearplug 50, or receiving communications from the transmitter though themagnetic field B exciting the wireless communications earplug 50.

If the invention is used in a noise defending communications headset,double hearing protection can be achieved: one layer of protection beingprovided by headset ear cups (not shown) and the other layer beingprovided by the noise attenuating properties of the wirelesscommunications earplug 50. The transmitter speaker 40 can be activatedif the user loses a wireless communications earplug 50, providing backupcommunications. Alternatively, the transmitter speaker 40 can be activeat all times.

Grounding the backing plate 22 using a grounding conductor 74 canprotect a wireless communications earplug 50 from electromagneticinterference, because the earplug 50 is generally used in closeproximity to the perforated backing plate 22. The shielding alsoprotects the coil 2 from stray electromagnetic pickup. In addition, ifelectronics are employed within a headset ear cup, the perforatedbacking plate 22 provides both magnetic and electrical shielding for theelectronics. Additional shielding may be used in a headset or helmet bylining the earcups and/or helmet with electrically conductive materialand/or magnetic material.

The magnetic field in front of the transmitters shown in FIGS. 1 through9 and within the inner radius R1 of the coils is fairly uniformespecially within the first centimeter of the transmitters. Hence, whena wireless earplug 50 is used with the transmitters, the sensitivity ofthe wireless earplug 50 and sound pressure produced in a human ear canalwill not vary appreciably so long as the wireless earplug 50 is withinthe inner radius R1 of the coil 2. Employing a larger coil 2 creates alarger region of uniform magnetic field; however, greater imaginarypower is required, as described earlier. An appropriated range for theinner radius R1 of the coil is between 0.5 in (12.7 mm) and 1.25 in (32mm) when the transmitter is used with a wireless earplug and headset.This yields coil circumscribed areas (πR1 ²) of between 0.79 in² (509mm²) and 4.9 in² (3167 mm²) The inventor has found from measurements inthe laboratory that this range of inner radius R1 provides reasonableuniformity of magnetic field. An inner radius R1 closer to 0.5 in (12.7mm) may be used for tighter fitting earmuffs and helmets, whereas aninner radius R1 closer to 1.25 in (32 mm) may be more appropriate forlooser fitting earmuffs and helmets.

Geometries other than circular geometries may be employed for the coil,such as rectangular, oval or others. The coil circumscribed areas ofthese geometries should be between 0.79 in² (509 mm²) and 4.9 in² (3167mm²) for reasonable efficiencies and coverage when incorporating awireless earplug in a headset. Moreover, non-planar geometries may beemployed.

Transmitter coils may be wound on bobbins to facilitate the windingprocess. Bobbins are commonly made of plastic or other rigid material.Magnetic backing plates may be mounted to such bobbins if desired.

Accordingly, it is to be understood that the embodiments of theinvention herein described are merely illustrative of the application ofthe principles of the invention. Reference herein to details of theillustrated embodiments is not intended to limit the scope of theclaims, which themselves recite those features regarded as essential tothe invention.

1.-23. (canceled)
 24. A magnetic field transmitter generating a timevarying magnetic field in response to a time varying input signal,comprising: a) a transmitter coil winding constructed of continuousconductor loops, having an inner perimeter and an outer perimeterseparated by a distance, the transmitter coil winding having a coilwinding resistance R and a coil winding inductance L, the innerperimeter of the loops of the transmitter coil winding, the coil windingresistance and the coil winding inductance being selected such that: i)the inner perimeter circumscribes an effective coil area in a rangebetween 0.79 in² and 4.9 in²; ii) the coil winding resistance and thecoil winding inductance produces a break frequency f_(b) for thetransmitter coil winding of between 20 Hz and 2000 Hz, where the breakfrequency is defined by the formula f_(b)=R/(2πL) wherein the coilwinding resistance R is defined by the formula R=ρ1/A and the coilwinding inductance L is defined by the formula L=N² (u_(o) πRe)/2; andiii) the loops of the transmitter coil winding having an annulus widthand an annulus depth which are each smaller compared to a distance froma center of the loops of the transmitter coil winding to the innerperimeter of the loops of the transmitter coil winding; and b) a backingplate of low reluctance magnetic material having an upper surface and alower surface, the lower surface of the backing plate being attached inclose proximity to a lower surface of the transmitter coil winding, thebacking plate having an outer perimeter that is approximately coincidentwith the outer perimeter of the transmitter coil winding.
 25. Themagnetic field transmitter of claim 24, in which the backing plate isperforated with at least one perforation.
 26. The magnetic fieldtransmitter of claim 24, in which the backing plate is cup-shaped,comprising a rim with an inner surface and an outer surface, the innersurface formed in close proximity to the outer perimeter of thetransmitter coil winding, extending from the upper surface of thebacking plate to the upper surface of the transmitter coil winding. 27.The magnetic field transmitter of claim 24, in which the transmittercoil winding has a front surface and a back surface separated by anannular depth L1, further comprising a magnetic core substantiallyfilling a volume defined by the inner perimeter of the transmitter coilwinding and the annular depth L1.
 28. The magnetic field transmitter ofclaim 27, where the magnetic core is perforated with at least oneperforation.
 29. The magnetic field transmitter of claim 27, in whichthe magnetic core is grounded.
 30. The magnetic field transmitter ofclaim 24, further comprising an audio acoustic speaker integrated withthe magnetic field transmitter to produce an acoustic output from theinput signal.
 31. The magnetic field transmitter of claim 24, furthercomprising a wireless ear plug communications device magneticallycoupled with the magnetic field transmitter to produce an acousticoutput from the input signal at the earplug.
 32. The magnetic fieldtransmitter of claim 24, in which the loops of the transmitter coilwinding are circular.
 33. A magnetic field transmitter generating a timevarying magnetic field in response to a time varying input signal,comprising: a) a transmitter coil winding constructed of a bundle ofcontinuous conductor loops, having an inner perimeter and an outerperimeter separated by a distance, an annulus width and an annulusdepth, the annulus width and the annulus depth being smaller compared toa distance from a center of the loops of the transmitter coil winding tothe inner perimeter of the loops of the transmitter coil winding thecoil winding having a coil winding resistance and a coil windinginductance which produces a break frequency f_(b) for the transmittercoil winding of between 20 Hz and 2000 Hz, where the break frequency isdefined by the formula f_(b)=R/(2πL), wherein the coil windingresistance R is defined by the formula R=ρ1/A and the coil windinginductance L is defined by the formula L=N² (u_(o) πRe)/2; b) a backingplate of low reluctance magnetic material having a front side and a backside, the transmitter coil winding being fixedly attached in closeproximity to the front side of the backing plate, the backing platehaving an outer perimeter that is approximately coincident with theouter perimeter of the transmitter coil winding and a central areaperforated with at least one perforation passing through the uppersurface and the lower surface; and c) an audio acoustic speaker forproducing an acoustic output from the input signal, mounted adjacent tothe at least one perforation in the backing plate, such that soundgenerated by the speaker passes through the at least one perforation inthe backing plate.
 34. The magnetic field transmitter of claim 33, inwhich the inner perimeter of the transmitter coil winding circumscribesan effective coil area in a range between 0.79 in² and 4.9 in².
 35. Themagnetic field transmitter of claim 33, in which the transmitter coilwinding has a coil winding resistance R and a coil winding inductance L,the coil winding resistance and the coil winding inductance beingselected to produce a break f_(b) for the transmitter coil winding ofbetween 20 Hz and 2000 Hz, where the break frequency is defined by theformula f_(b)=R/(2πL).
 36. The magnetic field transmitter of claim 33,where the magnetic backing plate is cup-shaped, comprising a rim with aninner surface and an outer surface, the inner surface formed in closeproximity to the outer perimeter of the coil winding, extending from thefront side of the backing plate to the upper surface of the transmittercoil winding.
 37. The magnetic field transmitter of claim 33, in whichthe transmitter coil winding has a front surface and a back surfaceseparated by an annular depth L1, further comprising a magnetic coresubstantially filling a volume defined by the inner perimeter of thecoil winding and the annular depth L1, the magnetic core beingperforated by at least one perforation aligned with the at least oneperforation of the backing plate.
 38. The magnetic field transmitter ofclaim 37, in which the magnetic core is grounded.
 39. The magnetic fieldtransmitter of claim 33, further comprising a wireless ear plugcommunications device magnetically coupled with the magnetic fieldtransmitter to produce an acoustic output from the input signal at theearplug.
 40. The magnetic field transmitter of claim 33, in which theloops of the transmitter coil winding are circular.
 41. The magneticfield transmitter of claim 33, in which the speaker is in a speakerhousing mounted to the back side of the backing plate, the housingproviding a separation space for a speaker diaphragm to vibrate withoutcontacting the backing plate.